Voxel-Based Analysis of Quantitative Multi-Parameter Mapping (MPM) Brain Data for Studying Tissue Microstructure, Macroscopic Morphology and Morphometry John Ashburner Wellcome Trust Centre for Neuroimaging, UCL Institute of Neurology, London, UK.

ROI Analyses The most widely accepted way of comparing image intensities is via region of interest (ROI) analyses. Involves manual placement of regions on images. Compute mean intensity within each region.

Automating ROI Analysis via Image Registration If all images can be aligned with some form of template data, ROIs could be defined in template space.

Automating ROI Analysis via Image Registration These ROIs could then be projected on to the original scans. Automatic. – Less work. – Repeatable. Needs accurate registration.

ROI Analysis via Spatial Normalisation Alternatively, we could warp the images to the template space. Use same ROI for each spatially normalised image. This naïve approach does not give the same mean ROI intensity as projecting ROIs on to the original images.

Expansion & Contraction DeformationsJacobian determinants

Weighted Average We can obtain the same results by using a weighted average. Weight by Jacobian determinants.

Weighted Average Jacobian scaled warped imagesJacobian determinants

Circular ROIs Circlular ROIs in template spaceCirclular ROIs projected onto original images

Convolution Original image After convolving with circle

Local Weighted Averaging Jacobian scaled warped imagesJacobian determinants

Local Weighted Averaging Smoothed Jacobian scaled warped imagesSmoothed Jacobians

Compute the Ratio Divide the smoothed Jacobian scaled data by the smoothed Jacobians. Gives the mean values within circular ROIs projected onto the original images. Ratio image

Gaussian Weighted Averaging We would usually convolve with a Gaussian instead of a circular function. Ratio image Gaussian kernel Circular kernel

Tissue-specific Averaging Smoothed data contains signal from a mixture of tissue types. Attempt to average only signal from a specific tissue type. Eg. White matter JE Lee, MK Chung, M Lazar, MB DuBray, J Kim, ED Bigler, JE Lainhart, AL Alexander. A study of diffusion tensor imaging by tissue-specific, smoothing-compensated voxel-based analysis. NeuroImage 44(3): , 2009.

Tissue-specific Averaging Original dataTissue mask

Masking the Data Masked dataTissue mask

Jacobian Scaling and Warping Jacobian scaled warped masked dataJacobian scaled warped mask

Smoothing Smoothed scaled warped masked dataSmoothed scaled warped mask

Compute the Ratio Gives the local average white matter intensity. Note that we need to exclude regions where there is very little WM under the smoothing kernel.

Problems/Challenges Needs very accurate image registration and segmentation. – Signal intensity differences of interest will bias segmentation/registration. Issues with partial volume – White matter signal may be corrupted by grey matter at edges. – Intensities dependent on surface area of interfaces.

Some Other Approaches JAD Aston, VJ Cunningham, MC Asselin, A Hammers, AC Evans & RN Gunn. Positron Emission Tomography Partial Volume Correction: Estimation and Algorithms. Journal of Cerebral Blood Flow & Metabolism 22(8): , A framework to analyze partial volume effect on gray matter mean diffusivity measurements. NeuroImage 44(1): , TR Oakes, AS Fox, T Johnstone, MK Chung, N Kalin & RJ Davidson. Integrating VBM into the general linear model with voxelwise anatomical covariates. Neuroimage 34(2):500–508, DH Salat, SY Lee, AJ van der Kouwe, DN Greve, B Fischl & HD Rosas. Age-associated alterations in cortical gray and white matter signal intensity and gray to white matter contrast. NeuroImage 48:21–28, SM Smith, M Jenkinson, H Johansen-Berg, D Rueckert, TE Nichols, CE Mackay, KE Watkins, O Ciccarelli, MZ Cader, PM Matthews & TEJ Behrens. Tract-based spatial statistics: Voxelwise analysis of multi-subject diffusion data. NeuroImage 31(4): , 2006.